1. Field of Invention
Separation of a mixture containing volatile components is a very important operation in chemical industries. In some cases, the required product purities are very high. Examples are production of polymerization grade monomers, such as ethylene, propylene, styrene, butadiene and p-xylene. Distillation operations have been used in most cases and freezing operations have only been used in some special cases. The distillative freezing process disclosed combines the advantageous features of both the distillation and freezing operations and remove their disadvantages.
2. Brief Description of the Prior Art
When a distillation operation is used in separating a mixture containing components that are very close in boiling points and volatilities, the number or plates required in very large and a large reflux ratio has to be used. Therefore, both the equipment cost and operating cost are very high. Furthermore, these costs increase greatly as the required product purity increases. Separations of ethylene-ethane mixtures, propylene-propane mixtures, styrene-ethylbenzene mixtures and p-xylene-m-xylene mixtures in producing high grade ethylene, propylene, styrene and p-xylene respectively are good examples. There is a great need for finding a better and more economical way of accomplishing the desired separations.
In a conventional fractional solidification process, a feed containing a crystallizing component and one or more impurities is brought into a two phase solid-liquid region to form a mixture containing crystals of the crystallizing component and a liquid mixture containing the impurities. An expensive scraped surface freezer is usually used in this operation. A centrifuge or a hydraulic washing column is then used to free the crystals from the mother liquor. Even though the crystals formed are usually very pure, it has been very difficult to produce a very high purity product of the crystallizing component. The difficulty arises because all the impurities in the feed are present in the liquid phase that surrounds the crystals and a complete separation of the crystals from the surrounding impure liquid phase is difficult. For example, crude p-xylene has been purified by a fractional solidification process. However, it has been difficult to produce p-xylene that is better than 99.9% in purity due to the difficulty described. Equipment cost of a conventional fractional solidification process is usually very high.
It is important to distinguish the distillative freezing process of the present invention from a vacuum freezing desalination process and a vacuum crystallization process. In a vacuum freezing desalination process (also called an evaporative freezing process), only one component (water) vaporizes and the same component (water) freezes. In a vacuum crystallization process, only one component (solvent) evaporates while the other component (solute) crystallizes. In a distillative freezing process two or more components are vaporized from a feed under a sufficiently reduced pressure and only one component freezes. The low pressure vapor formed in a distillative freezing process is a vapor mixture and can be transformed to a condensed mass in a simple way without pressurization.